1. Field of the Invention
This invention relates to a process and a composition for producing surfaces that are self-cleaning by water. In particular, the invention relates to an aqueous composition for forming self-cleaning surfaces.
2. Description of the Related Art
It is reported in U.S. Patent Application Publication No. 2002/0048679 (and related European Patent Application No. EP 1018531) that surfaces from which water runs off easily have to be either very hydrophilic or hydrophobic. Hydrophilic surfaces have low contact angles with water, and this brings about rapid distribution of the water on the surface and finally rapid run-off of the resultant film of water from the surface. In contrast, hydrophobic surfaces form droplets through large contact angles with water. These droplets can roll off rapidly from inclined surfaces.
U.S. 2002/0048679 also notes that articles with surfaces which are (or have been made) difficult to wet have a number of commercially important features. For example, these surfaces are easy to clean, and deposits find it difficult to adhere to these surfaces. Thus, there have been numerous efforts to create surfaces which are difficult to wet in order to provide dirt-repellent surfaces and/or surfaces that can be self-cleaned by rain or moving water. U.S. 2002/0048679 cites surfaces having a smooth extremely hydrophobic polymer (e.g., polytetrafluoroethylene) film and surfaces having a smooth extremely hydrophilic polymer film as examples of surfaces on which water and dirt can run off without forming droplets. U.S. 2002/0048679 teaches that a long-term hydrophobic coating can be formed by applying certain silane derivatives underneath a hydrophobic coating on a surface. Other self-cleaning surfaces are described in U.S. Patent Application Publication Nos. US 2002/0150723, US 2002/0150724, US 2002/0150725, US 2002/0150726, US 2003/0013795 and US 2003/0147932.
UK Patent Application GB 2121059 describes the problems associated with the staining of the glass used in greenhouses. In particular, stained glass causes a loss in light transmission that results in a loss of yield in a crop such as tomatoes. This reference teaches that coating compositions formed from a hydrophilic monomer (e.g., hydroxyalkyl acrylate) and a hydrophobic monomer (e.g., an alkyl acrylate) can form a self-cleaning coating on a glass surface. The coatings are said to be self-cleaning (e.g., by rain) yet sufficiently water-resistant to be durable on outdoor exposure.
U.S. Pat. No. 5,324,566 describes the use of hydrophobic fluorinated siloxane polymers for producing water repellent surfaces. It is disclosed in this patent that the water repelling properties of the fluorinated siloxane polymer surface film can be improved by forming surface irregularities in the surface and/or the surface film. In one form, the surface is provided with irregularities of a height from about 0.1 micrometers up to the wavelength of visible light. See, also, U.S. Pat. No. 5,599,489.
U.S. Pat. No. 6,156,409 describes the use of surface irregularities in a film coating to create hydrophilic surfaces having low contact angles with water and having improved water run-off. In one embodiment, the film is formed so as to have an arithmetic mean roughness (Ra) from 1.5 to 80 nanometers and a mean interval (Sm) of dents and projections from 4 to 300 nanometers by using metal oxide fine particles in a coating solution. It is noted that if the mean interval (Sm) value exceeds 300 nanometers, the transparency of the coating may be spoiled.
U.S. Pat. No. 3,354,022 also discloses methods for preparing water repellent surfaces wherein surface projections or surface depressions are formed in a surface. CH-PS 26 82 58 describes water repellent surfaces which have a contact angle of over 120 degrees with water. The surfaces are obtained by application of powders such as Kaolin, talc, clay or silica gel onto a substrate, where the powder is first made hydrophobic by means of organic silicon compounds. The application takes place together with hardenable resins or from solutions in organic solvents.
WO 96/04123 and related European Patent No. EP 772514 B1 describe other processes for lowering the wettability of articles via topological changes to the article surfaces. The applicant in WO 96/04123 reported that after many years of experimentation, it was determined that surfaces of certain plants are capable of being cleaned by rain or moving water and that no solid dirt particles can be permanently deposited on such surfaces. By way of various studies, the applicant in WO 96/04123 determined that the ability for self-cleaning of a plant surface is very dependent on the surface structure of the plant. The applicant disclosed that it is technically possible to make the surfaces of articles artificially self-cleaning, merely by providing them artificially with a surface structure of elevations and depressions, where care is taken such that the distance between the elevations of the surface structure in a range of 5 to 200 micrometers, the height of the elevations is in the range of 5 to 100 micrometers, and the elevations consist of hydrophobic materials. The described self-cleaning surfaces can be produced either by creating the surface structures from hydrophobic polymers right during the manufacture, or creating the surface structures subsequently, and specifically either by subsequent imprinting or etching or by adhesion of a polymer made of the hydrophobic polymers. Often, the self-cleaning surfaces described in WO 96/04123 are termed “Lotus-effect” surfaces or coatings, and the technology is termed “Lotus-Effect” technology.
European Patent Application No. EP 933388 describes articles having surfaces with liquid-repellent regions having projections having a mean height of 50 nanometers to 10 micrometers and a mean spacing of 50 nanometers to 10 micrometers. European Patent Application No. EP 1040874 (also Canadian Patent Application 2,302,118) describes articles having surfaces which have regions that are liquid-repellent and regions that are liquid-wetting in a defined location. The liquid-repellent regions have protuberances having a mean height of 50 nanometers to 10 micrometers and a mean spacing of 50 nanometers to 10 micrometers. European Patent Application No. EP 937962 describes methods for examining these types of surfaces.
While the above documents teach various methods for creating self-cleaning surfaces (or “Lotus-Effect” surfaces) that combine surface roughness and/or hydrophobicity to provide self-cleaning benefits, the reported methods and materials do have drawbacks. Specifically, the reported “Lotus-Effect” technology suffers from several key limitations including incompatibility with aqueous formulations, and the formation of coatings that are opaque white and exceedingly fragile. Thus, there is a need for a process and an aqueous system for producing surfaces that are self-cleaning by water and transparent.